1. Field of the Invention
The present invention relates to a method and an apparatus to control a data storage system, and more particularly, to a method and an apparatus to restore the stability of a head in a data storage system by automatically applying a bias shock current to the head in accordance with a result of a thermal asperity detection (TAD) method, in the case where a quality problem occurs in the manufacturing process and/or under severe user conditions.
2. Description of the Related Art
An existing inductive head uses a method of winding a head core with a coil to transform changes in magnetic flux of a disk into changes in a voltage of a head coil. In the inductive head, the inductance of the head coil has to be lowered for the stability of a data signal if the frequency of the data signal becomes high. This lowers the induction voltage of the head and thus makes the detection of a data signal unstable.
A magneto-resistive head (hereinafter referred to as “MR head”) has been developed in response to the problem of the existing inductive head. The MR head uses an MR sensor, which easily senses changes in magnetic flux, to adapt to a data signal having high frequency. Unlike the existing inductive head in which the head coil transforms changes in magnetic flux into changes in the induction voltage, when the MR head reads data, the MR head uses the MR sensor to detect changes in magnetic flux as changes in a resistance value. However, the data storage system uses the existing inductive head structure to improve the writing density per unit area and the signal-to-noise (S/N) ratio in the writing process.
However, the MR head, unlike the existing inductive head, causes an instability of the MR sensor due to an unstable functioning domain of the MR sensor produced by soft electrostatic discharge (ESD) and an inappropriate hard bias operation of both ends of the MR sensor.
A reduction in the width of the MR sensor with an increase in the track per inch (TPI) further increases the instability of the MR head if the capacity of the MR sensor is increased to increase the sensitivity per area of the MR sensor.
In a disk drive, the instability of the MR head causes baseline popping noises, amplitude spikes, and amplitude asymmetry of a signal. As a result, the bit error rate (BER) becomes poor or an error occurs in an error correction code (ECC) inspection when reproducing data. Moreover, the instability of the MR head may occur anytime even in a normal state, and thus increases a defect rate in the manufacturing process and affects the quality of products on the market.
U.S. Pat. No. 5,650,887 and Korean Laid-open Patent No. 1999-72284 relate to a technique to restore the stability of an MR head. U.S. Pat. No. 5,650,887 discloses a technique to apply a reset current to an MR head only if measured servo automatic gain control (AGC) is less than 20%. Korean Laid-open Patent No. 1999-72284 discloses a technique to determine the performance of a head depending on whether a servo signal is detected to apply an inverse reset current to the head.
The instability of the MR head causes a quality problem in processing data in that data error and BER exceeds a reference value. In this case, the instability of a head is not conventionally determined by the baseline popping noise directly caused by the instability of the MR head. Thus, it cannot be accurately determined whether the quality problem occurring in processing data is due to the instability of the MR head in the manufacturing process or a user's condition. As a result, the instability of the head cannot be restored well.